A multi-cell power converter is here to be understood as a power converter comprising a series connection of at least two identical subunits, called cells, where each subunit or cell comprises at least four power electronic valves and an energy storage unit, such as a capacitor. The series connection of the at least two cells can be connected either between two AC phases or between an AC phase and a neutral or between a first and a second DC pole, depending on the nature of the power converter, i.e. depending on whether it is an AC/AC or AC/DC converter, on how many phases it possesses and on how the phases are connected to each other.
Each of the cells of the multi-cell power converter, which the invention is related to, comprises four power electronic valves interconnected as a full-bridge converter or sub-converter with a cell capacitor connected in parallel to two identical phase legs, each phase leg comprising a series-connection of two of the power electronic valves. Such a cell structure used for an AC/AC converter is for example known from F. Z. Peng et al, “A multilevel voltage-source inverter with separate DC sources for Static Var generation”, IEEE Transactions on Industry Applications, Vol. 32, No. 5, September/October 1996, pp. 1130-1138. The connection point between the two power electronic valves of each of the two phase legs is connected to an output terminal, i.e. the cell is via these two output terminals connectable to either another, neighbouring cell or to any other external device or connection.
The power electronic valves of the converter the invention is related to each comprise a power semiconductor switch of turn-off type, such as an IGBT (Insulated Gate Bipolar Transistor), an IGCT (Integrated Gate Commutated Thyristor) or a GTO (Gate Turn-off Thyristor), and a free wheeling diode in anti-parallel connection thereto. The power semiconductor switch and the free wheeling diode may be singular devices or a series-connection of multiple such devices, depending on the voltage levels they are intended to be used for. The same is true for the cell capacitor.
In order to generate the electronic signals required to turn each of the power semiconductor switches on and off, a so called gate driver unit or gate unit is connected to the gate of each power semiconductor switch. The structure and function of an example for such a gate unit is known from EP0868014B1, where the gate unit receives via a fibre optic communication line a control signal from a control apparatus. The control signal is either a logical one to turn the corresponding power semiconductor switch on or a logical zero to turn it off.
The four power semiconductor switches in the power electronic valves of each cell of the multi-cell power converter as described above are according to the art controlled in such a way that one of three different possible voltage levels is generated across the output terminals of the corresponding cell, where the voltage levels are either plus or minus the voltage across the cell capacitor or a zero voltage. The voltage across the cell capacitor is in the following called the cell capacitor voltage and a numerical representation of the cell capacitor voltage is called cell capacitor voltage value. By appropriately timing the switching of the power semiconductor switches, it is in addition ensured that the cell capacitors are recharged at least from time to time in order to compensate for any losses in the respective cell. Furthermore, the cell capacitors are recharged or charged to keep the cell capacitor voltage on a desired capacitor voltage level which is preferably equal for all series-connected cells in order to keep the voltage division over the cells balanced. The overall voltage of the multi-cell power converter is then controlled by adjusting the sum of the output voltages of all series-connected cells in a desired manner.
For this control of the cells and the converter, respectively, the individual cell capacitor voltage values need to be known. A straightforward solution to obtain a cell capacitor voltage value is to arrange a first voltage measurement unit in parallel connection to the cell capacitor.